Melatonin receptors: We characterized high affinity melatonin receptors in cerebral nuclei and in brain and caudal arteries involved in temperature control These receptors are coupled to guanine nucleotide binding proteins. Melatonin modulates caudal artery contractions. Angiotensin receptors: Brain angiotensin receptors are more widely expressed in developing rats than in mature animals, and they are selectively modulated by specific ion deficiencies and by the extracellular fluid volume. The receptors can be classified using autoradiography and displacement with new specific receptor inhibitors. Beta-adrenoceptors: Beta(2)-adrenoceptors control the regulation of cortical serotonin 2 during stress. Peripheral sympathetic ganglia contain only Beta(2)-adrenoceptors, and the receptor number is high in genetically hypertensive rats. Methodology: We developed new sensitive autoradiographic radioimmuno-- histochemical methods for the quantification of tissue antigens (neuropeptides, peptidases, amine receptors) in selected brain areas, and in human platelets. These methods have higher-over 50 fold-sensitivity than classical methods and will allow new pharmacological and clinical studies.